1-oxa .beta.-lactam compounds, which possess the following general structure: ##STR1## have recently been the subject of extensive research efforts due to their antibacterial activity. Specifically, there have been some recent reports of syntheses of 1-oxa .beta.-lactam compounds substituted with methyl at the C-3 position. For example, Naylor et al. in "Recent Advances in the Chemistry of .beta.-Lactam Antibiotics" (J. Elks, ed.), The Chemical Society, London, 1977, p. 204, reported a synthesis of 1-oxacephalexin. Similarly, Narisada et al., Heterocycles, 7, 839 (1977), were able to prepare several 3-methyl 1-oxa .beta.-lactam compounds which exhibited antibacterial activity from four to eight times greater than the corresponding cephalosporins. The present application describes and claims novel intermediates which can be employed in the synthesis of the aforementioned biologically active 7.beta.-acylamino-7.alpha.-alkoxy-3-methyl 1-oxa .beta.-lactam compounds. The process for the conversion of such intermediates into other unclaimed intermediates in the synthesis of the aforementioned 1-oxa .beta.-lactam antibiotics is an alternate aspect of this invention.